In the field of Medical Imaging, one modality is Nuclear Medicine (gamma camera, SPECT and PET) imaging. This modality uses a detector consisting of a scintillator backed by a plurality of photomultiplier tubes (PMTs) with appropriate electronics. A patient is given a radioisotope either by injection or ingestion and the detector(s), after being placed in close proximity to the patient, can determine where the radioisotope goes or has gone.
The process of detection is when the radioisotope emits a gamma photon in the direction of the detector; it is absorbed by the scintillator. The scintillator emits a flash of light (a scintilla) which is detected by the plurality of PMTs. The PMTs closer to the flash have a higher signal than those further away. By measuring the intensity of the flash at each PMT, then using a centroid type calculation, a fairly accurate estimation of where the flash occurred is possible. All this is well known in the art.
During the process of image reconstruction in a SPECT or PET system, correcting for the probable attenuation of the gamma photons is desirable. When corrected for attenuation, images are much more accurate and less prone to diagnostic errors.
To image accurately some type of collimation is needed. Traditionally, a parallel hole collimator is used. This typically allows only gamma rays traveling perpendicular to the face of the detector, to be detected. Other gamma rays, traveling obliquely to the face of the detector, are typically absorbed by the lead in the collimator.
Alternate types of collimators have been used for different types of studies. For example, a pinhole collimator is sometimes used to image specific organs such as the thyroid. The principle behind a pinhole collimator is similar to a pinhole camera or camera obscura, i.e., only photons traveling through the pinhole strike the detector. An advantage of a pinhole collimator is it can achieve high magnifications with high resolution. A disadvantage is because photons can only travel through the pinhole, the sensitivity of the system can be poor.
Another type of collimator is a fan beam collimator. This type of collimator is used to acquire fan beam type data for use in fan beam reconstructions. It can again achieve magnification (or demagnification), but typically only in one dimension, i.e., the direction of the fan beam.
Yet another type of collimator is a cone beam collimator. A cone beam collimator can be either converging or diverging. A converging cone beam collimator is a demagnifier allowing viewing of larger objects using a smaller detector. A diverging cone beam collimator is a magnifier allowing better visualization of small objects.
A problem for typical collimators is sensitivity. Because collimators essentially reject any gamma photons which are not parallel to the holes or apertures in the collimators, a large percentage of photons traveling in the general direction of the detector are absorbed by the collimator and not detected for use in the images. While this may allow good images to be generated, it can take significant time to detect enough photons to generate a good low noise image.
Another problem for collimators is due to the optics of the collimator; the resolution of collimator-detector system deteriorates the further the object is from the face of the collimator.
It would be useful to improve the sensitivity of a collimator and improve the resolution of a collimator, especially at significant distance from the detector.
U.S. Pat. Nos. 6,525,320; 7,012,257; 7,015,476; 7,071,473; all incorporated herein by reference, describe using a stationary collimator with multiple slots in front of a large, stationary, arcuate detector. This typically allows for no space or method for acquiring attenuation correction data in the SPECT system. In addition, the slot of the collimator moves in relation to the detector. This typically requires having a large, expensive detector behind the collimator. Economically, the detector is typically the expensive component in the assembly. The collimator is typically relatively inexpensive. It would be more economical to have smaller detectors, each with its own collimator. In addition, since the detector is one continuous arc, there is typically no place to put a co-planar CT type system for generating attenuation correction maps.